1. Field of the Invention
The present invention generally relates to systems and methods for correcting scan position errors in an imaging system.
2. Background Description
FIG. 1, generally at 100, is an illustration of a known flat field scanning system in which images can be plotted on a suitable medium, such as photosensitive material 132, from an electrical signal, such as an externally applied video signal 90 of the image. Video signal 90 modulates a power source such as laser driver 95, which, in turn, drives light source 110. Light source 110 converts video signal 90 into an image modulated visible or infrared light output that is collimated by lens 112 to form an image modulated light beam 115. Image modulated light beam 115 may be visible, infrared, or ultraviolet light and is generated by a laser, such as a helium neon laser or a semiconductor laser diode. Image modulated light beam 115 is deflected by a rotatable mirror 120 having, for example, a 45 degree surface 150, towards a focusing lens (or scan lens) system 125, which focuses deflected light beam 145 into an image point on photosensitive material 132. A high-speed motor 142 drives rotatable mirror 120 about a rotation axis 140.
As mirror 120 is rotated, beam 145 passes through scan lens system 125, causing a focused spot to move in a raster-like fashion along an imaging line 135 on material 132. The scan angles 0 that are swept out during imaging by the surface 150 span a range from approximately −32° to +32°. During this period, information contained in beam 145 exposes photosensitive material 132 in a sweep or scan-like manner. To produce the sweeping action of beam 145, motor 142 rotates mirror 120 at a pre-determined angular velocity. For a high-resolution scan, imaging line 135 is very fine (e.g., less than about 1/1000 of an inch wide). To scan an image field rapidly with such fine scan-beams, motor 142 typically turns mirror 120 at a high frequency (e.g., 20,000 revolutions per minute (RPM)).
Images are plotted by repetitive deflection of beam 145 where, for example, imaging line 135 is plotted in one beam deflection. Images are plotted as a successive plotting of imaging lines 135, wherein each imaging line 135 is made up of image elements known as pixels. More particularly, to expose the second dimension of photosensitive material 132, photosensitive material 132 can be translated in a direction perpendicular to imaging line 135 using techniques known in the art, such as a standard capstan roller. Alternatively, focused beam 158 can be translated perpendicular to imaging line 135 on photosensitive material 132 by, for example, using another movable mirror (not shown) positioned between scan lens system 125 and photosensitive material 132 to redirect beam 158. In addition, scan lens system 125 can be translated in a direction perpendicular to imaging line 135 using techniques known in the art, such as a flat bed Scan lens system 125 is constructed and arranged to focus beam 145 during scanning at all points along imaging line 135. In particular, scan lens system 125 can be an f-theta lens, i.e., it maintains the relationship Y=f×θwhere f is the effective focal length of the system, 74 is the scan angle, and Y is the distance of the imaged object along imaging line 135 from optical axis 98. An f-theta lens ensures that the scanning speed of beam 145 across the flat image field on photosensitive material 132 is uniform for a constant angular velocity of rotatable mirror 120.
In a scan lens system, such as scan lens system 125, that uses an f-theta lens, position errors occur when image pixels are not located at their ideal position on photosensitive material 132. For example, on imaging line 135, a pixel intended for position 135a may actually appear at position 135b. The distance between the actual position 135b and the intended position 135a is the position error. Sources of position error(s) can include: (a) scan lens system 125 design and/or assembly; (b) scan lens system 125 tilt (the scan lens system 125 is not ideally located in the path of beam 145); and (c) contributions of other optical and/or mechanical components positioned in the laser beam path that have non-ideal characteristics, and/or a non-ideal location of other optical and/or mechanical components in the laser beam path.
While position error is generally predictable for a given scan lens system 125 design, the actual position error produced by the scan lens system 125 can deviate from the predicted position error due, for example, to lens manufacturing tolerances and/or variations in other system components and alignments. It should be understood that position errors can also occur, for example, in drum type imaging systems, where such position errors are typically caused because the drum surface, or sections thereof, are not ideally located at the laser beam path.
One or more embodiments of the present invention is directed to systems and methods for reducing or eliminating position errors that occur in imaging systems.